Light microscopes hit a wall at about 200 nanometres, however much they magnify
You can keep cranking up a light microscope's magnification, but past a certain point the picture just gets bigger and blurrier. Because visible light diffracts, details closer together than roughly 200 nanometres merge into one. To see smaller, scientists abandon light for electron beams or scanning probes.
Optical microscopes are the oldest kind, with today's compound form dating from the 17th century. A simple microscope, such as a magnifying glass or loupe, uses one lens or lens group and shows an upright, enlarged virtual image. A compound microscope uses two stages. The objective lens near the specimen forms a real image inside the tube, and the eyepiece then magnifies that image again, which is why the view appears upside down. Multiplying the two stages gives far higher magnification than one lens alone.
Most research instruments are compound, usually with several objectives on a rotating turret so the user can switch power quickly. Some cheaper digital microscopes, by contrast, are simple single-lens designs. A camera can be attached to record the image, which is then called a micrograph.
Lighting matters as much as lenses. Transparent specimens are usually lit from underneath. Bright field lighting sends light through the objective, while dark field lighting comes in around it, making specimens glow against black. Polarised light reveals the orientation of crystals in metals, and phase contrast picks out tiny differences in refractive index to make faint, clear structures visible.
Specialised designs abound. Stereo microscopes feed each eye a slightly different image for a three-dimensional view and are common for dissection. Comparison microscopes put two samples side by side, one per eye. Inverted microscopes look up from below, handy for cells growing in liquid or for examining metals. Petrographic microscopes add a polarising filter, a rotating stage and a gypsum plate so geologists can study minerals whose optical behaviour changes with orientation, and dedicated models exist just for inspecting the polished ends of fibre-optic connectors.
Source: Optical microscope
